1. Field of the Invention
The present invention generally relates to a method for generating a servo signal of an optical storage system, and more particularly, to a method for generating a radio frequency ripple zero crossing signal (RFZC signal) used as the base of track jumping and track following in an optical storage system.
2. Description of Related Art
In a common optical storage system, an RFZC signal in association with a tracking error zero crossing signal (TEZC signal) are used for servo controlling track jumping and track following of an optical pick-up head.
FIG. 1 is a block diagram of a conventional apparatus for generating an RFZC signal and FIG. 2 is a diagram showing the principle of generating an RFZC signal in a conventional apparatus. Referring to FIGS. 1 and 2, a radio frequency signal (RF signal) RF is received by a radio frequency envelope signal generating unit (RF-ENVLP signal generating unit) 102, a radio frequency envelope signal (RF-ENVLP signal) RF-ENVLP is generated by deducting the bottom hold level of RF signal from the peak hold level of the RF signal, or generated by taking the bottom hold level of radio frequency signal only. Then, an RFRP signal generating unit 104 is used to generate a radio frequency ripple signal (RFRP signal), wherein the RFRP signal is generated by calibrating the gain of the RF-ENVLP signal and performing a low-pass filtering (LPF) processing.
After that, a radio frequency ripple signal slicing level generating unit (RFRP signal slicing level generating unit) 106 is used to detect the central level of the RFRP signal so as to obtain a radio frequency ripple signal slicing level (RFRP signal slicing level) RFCT, wherein the common method for generating the RFRP signal slicing level is to use the average value of the peak hold level and the bottom hold level of the RFRP signal. When a DC offset of the RF signal is occurred, a signal calibration unit 108 is used to perform calibration on the RFRP signal, which is able to ensure the desired RF signal slicing level RFCT can be correctly generated based on the RFRP signal. In the end, a signal comparison unit 110 is used to compare the RFRP signal with the RFRP signal slicing level RFCT to generate the RFZC signal.
FIG. 3 is a diagram an abnormal RFZC signal caused by a DC offset of RF signal. Referring to FIG. 3, in the above-mentioned prior art, when a DC offset suddenly occurs with the RF signal, the RF-ENVLP signal would get level drift or saturation cutoff, which limits the compensation effect and makes the RFRP signal slicing level RFCT unable to correctly slice the RFZC signal (as shown by a doted line circle in FIG. 3).